Some automobiles are equipped with an apparatus for controlling the idling rotation speed, by comparing the engine rotation speed under idling condition with a target idling rotation speed, changing the opening of a throttle valve depending upon the deviation therebetween so that the engine speed is controlled at a target rotation speed, thereby reducing the consumption of fuel under the idling condition.
Conventional apparatuses for controlling the idling speed use a cheaply constructed DC motor that works as an actuator to change the opening of the throttle valve, the DC motor being controlled in a rotating direction which corresponds to a speed deviation polarity between a target rotation speed and a practical rotation speed. To improve the control precision, furthermore, the DC motor is intermittently driven by intermittent feedback control pulses of a predetermined period, and the pulse width thereof is controlled depending upon the amount of the speed deviation. In this conventional apparatus, the pulse period is set so that the pulse pause interval hereinafter referred to as hold time becomes relatively long, by taking into consideration a snaking time of the DC motor and the delay time between the time when the opening of the throttle valve is changed and the time when the change of the engine speed is reflected thereby. Therefore, when it is expected that the engine rotation speed changes or drops abnormally due to the operation of loads such as an air-conditioning apparatus or a power steering appratus during a predetermined hold time, it is necessary to wait for the next pulse even when an estimated correction according to a pulse width correction is to be effected by detecting the operation of such loads. Accordingly, response for the change in the engine load is delayed possibly causing the engine to stall.